1. Field of the Invention
This invention relates to a fuel injection system for an engine.
2. Description of the Prior Art
Recently electronic-control fuel injection systems have been in wide use in various vehicle engines. In such electronic-control fuel injection systems, so-called top-feed type fuel injection valves have been commonly used.
The top-feed type fuel injection valve is a solenoid-operated fuel injection valve and has a tubular valve body. In the top-feed type fuel injection valve, fuel is fed to the valve body through a feed port formed near the top end thereof and injected through an injection port formed on the bottom of the valve body. The top-feed type fuel injection valve having such a structure is advantageous in that since the fuel simply flows in the tubular body in the axial direction thereof, the injection valve may be simple in structure.
On the other hand, the top-feed type fuel injection valve is disadvantageous in the following point. That is, in the top-feed type fuel injection valve, the fuel return passage is provided upstream of the feed port, and accordingly, a certain amount of fuel dwells in the valve body. When the fuel dwelling in the valve body is evaporated by heat transmitted from the engine body or by radiation heat therefrom, vapor lock can occur.
Further, in some recent engines, the intake passage is long in order to increase the engine output by kinetic effect of intake air, and at the same time, a surge tank is disposed above the intake manifold and a cover is provided over the surge tank in order to compactly arrange the engine. In such engines, the fuel injection valves are more apt to be heated and the aforesaid problem is more serious.
In order to solve the problem, there has been proposed a bottom-feed type fuel injection valve as disclosed, for instance, in Japanese Unexamined Utility Model Publication Nos. 62(1987)-29469 and 62(1987)-193177. In the bottom-feed type fuel injection valve, fuel is fed to the valve through a feed port formed in a lower portion of the valve body and the residual fuel is returned to the fuel piping (generally the fuel feed piping for the next cylinder) through a fuel discharge port formed in the valve body at the substantially same level as the fuel feed port. Generally the bottom-feed type fuel injection valve comprises an outer casing, and an injector main body (formed of a solenoid coil and an on-off valve) which is housed in the outer casing, and the feed port and the fuel discharge port are formed in the lower portion of the outer casing. The solenoid coil is connected to a connector which is provided in a wire harness one for each cylinder.
However, various difficulties are encountered in actually incorporating the bottom-feed type fuel injection valves in the intake system of the engine and connecting them to the fuel piping and the power supply harness members. That is, unlike the top-feed type fuel injection valve, in the case of the bottom-feed type fuel injection valve, both the fuel feed port and the fuel discharge port are disposed in the lower portion of the outer casing and accordingly, the structure around the lower portion of the outer casing becomes complicated, thereby making it very difficult to provide means for fastening the valve and deteriorating the workability in incorporating the valves into the intake system.